Do merchants deserve a commission for the purchase of a plot of land, given that no prior agreement was made with them, and that the buyer reached the landowner through other means?

The basic ruling on brokerage is permissibility, and it falls under the category of ju'alah (commission/reward). Accordingly, the owner of the first office who directed you to the land, and the owner of the office who gave you the landowner's number, both deserve a fee for that, regardless of whether the landowner knew about it. The crucial point is their guidance to you regarding the land and its owner's number. As for the other offices, they are not entitled to a fee because you did not benefit from their guidance.